Thanks for the support Jonno.
Although the data is public a search of all followers and those follower's profiles can become quite intensive on large accounts.
If implemented, this metric may be provided on a less frequent basis (perhaps weekly).
We are implementing some functionality with follower tracking that could lend well to the second-tier followers feature.
We'll keep you posted.
Regarding launch date, nothing concrete to report, yet.
Always love the feedback. Thank you!

It really would be cool if this got implemented. It seems like it would be easy, all the data is publicly available through search. It's just the sum of all the people following all of the followers of any given account.